A definition of contagion was suggested by Sigal G. Barsade- "a process in which a person or group influences the emotions or behavior of another person or group through the conscious or unconscious induction of emotion states and behavioral attitudes".[1]
So, for example, emotional contagion is the tendency to express and feel emotions that are similar to and influenced by those of others. One view of the underlying mechanism is that it represents a tendency to automatically mimic and synchronize facial expressions, vocalizations, postures, and movements with those of another person and, consequently, to converge emotionally (Hatfield, Cacioppo, & Rapson, 1994).